a) Field of the Invention
The present invention relates to a semiconductor device and its manufacture method, and more particularly to a semiconductor device and its manufacture method capable of providing a low contact resistance between impurity doped regions in a surface layer of a silicon substrate and a wiring layer.
b) Description of the Related Art
A region heavily doped with impurities in the surface layer of a silicon substrate is electrically connected to a wiring layer of Al or Al alloy formed thereon by inserting a barrier metal layer therebetween for blocking solid diffusion of atoms in the substrate and in the wiring layer. The barrier metal layer improves the reliability of the contact area.
Known barrier metal materials are refractory metal silicide such as WSi.sub.2 and MoSi.sub.2 and refractory metal alloy such as TiW. In order to improve thermal endurance and barrier performance, a lamination structure of a refractory metal layer and a refractory metal nitride layer such as a lamination structure of TiN and Ti, and a lamination structure of HfN and Ti, has been recently used.
A contact resistance between a metal wiring layer and a semiconductor substrate depends on an impurity concentration of the semiconductor substrate and a work function of the metal. A contact resistivity .rho..sub.c is given by: EQU .rho..sub.c =C.times.exp(4.pi.(.epsilon..sub.s m.sup.*).sup.1/2 /(qh).times.(.phi..sub.B /N.sub.D) (1)
where .phi..sub.B is a Schottky barrier height at an interface between semiconductor and metal, N.sub.D is an impurity concentration of a semiconductor substrate, m.sup.* is an effective mass of carriers in the semiconductor substrate, .epsilon..sub.s is a dielectric constant of semiconductor material, q is an electronic charge, h is a Planck's constant, and C is a constant.
As seen from the equation (1), the contact resistance can be lowered by increasing the impurity concentration N.sub.D. However, impurities near a limit of solid solubility are doped presently and it is difficult to increase the impurity concentration higher than the present level.